1. Field of the Invention
The present invention relates to a back light device for a liquid crystal display. More specifically, the present invention relates to a back light device used for a transparent type lighting of a liquid crystal display adopted as a display for a mobile personal computer or other equipment.
2. Background Art
An example of the back light device for a liquid crystal display mentioned above in the prior art is disclosed in Japanese unexamined patent publication No. Hei 5-203947. In this publication, the back light device 90 includes a light guide plate 91 that is made of a transparent material such as an acrylic resin, and has a shape of substantially thick plate. One or each end side (not illustrated) of the thick plate is provided with a tubular light source 92 such as a cold cathode lamp that is disposed adjacent to the end side for emitting light into the light guide plate 91.
The back surface of the light guide plate 91 is provided with a reflecting surface 93 by applying a white paint, for example. The reflecting surface 93 works for directing the light to the front surface that has reached the back surface (reflecting surface 93) out of the light lead into the light guide plate 91 from the light source 92. The front surface of the light guide plate 91 is provided with a plurality of diffusing prisms 91a having a substantially isosceles triangle section and a ridge extending along the direction perpendicular to the axis Z of the light source 92.
The diffusing prisms 91a direct the light to the outside from the surface as a substantially diffused light that has reached the diffusing prism 91a directly from the light source 92 or has been reflected by the reflecting surface 93. However, diffusion effect thereof is not sufficient only by the diffusing prism 91a. Therefore, a diffusing sheet 94, prism sheet 95 or other members having diffusion effect is disposed between the light guide plate 91 and a liquid crystal display 80 located in the front side of the light guide plate 91. Thus, more diffused light passes through and illuminates the liquid crystal display 80.
However, in the above mentioned back light device 90 of the prior art, the diffusing prisms 91a formed on the surface of the light guide plate 91 have not been fully examined about its shape. For example, the apical angle of the isosceles triangle section of the diffusing prism 91a was set at 90 degrees for reasons such as an easiness for making a mold of the light guide plate 91. Therefore, it was a problem to be improved that the liquid crystal display was not always illuminated under the best condition.